Superalloys are materials, usually nickel or cobalt based, which have useful properties at temperatures ranging from cryogenic to approximately 2000.degree. F. (1093.degree. C.). Thus the superalloys are useful in applications ranging from cryogenic turbopumps to gas turbine engines.
Nickel base superalloys generally consist of a .gamma. (nickel solid solution) matrix containing an array of particles which contribute to the strength of the material. The strengthening particles generally include, but are not limited to, .gamma.', .gamma.", and carbides. These strengthening particles are referred to hereinafter as second phase particles.
In addition to providing a strengthening mechanism for the superalloys, the second phase particles also control the grain size of the alloys during various high temperature processing operations by controlling dislocation and grain boundary movement.
Superalloy castings used in the aerospace industry frequently contain both thick and thin sections. The as cast microstructure is frequently relatively fine grained (ASTM 3.5-6.0). The fine grained microstructure provides mechanical properties, such as yield strength, tensile strength, ductility, high cycle fatigue and low cycle fatigue, which are highly desirable for these aerospace applications. Welding of the castings, such as for repair of casting or machining defects, is also more successful with a fine grained microstructure.
During cooldown within the mold following casting, high residual stresses can arise due to the variation in cooling rate in the thin and thick sections, with the thin sections generally being subjected to tensile stresses.
Processing of the castings to optimize the mechanical properties typically includes a homogenization or solution heat treatment which is designed to reduce local chemical variations (due to solidification processes) and to dissolve some or all of the second phase particles. This is followed by subsequent heat treatments designed to reprecipitate the second phase particles to provide a particle size and particle distribution which will optimize the required mechanical properties.
The homogenization heat treatment, which involves dissolving some or all of the second phase particles, may remove the primary barriers to grain boundary movement and produce subsequent grain growth. Grain growth due to the combination of existing high residual stresses and diminished resistance to grain boundary movement results in a generally coarse grain microstructure with occasional grains which are extremely large. In some cases extremely large single grains can extend through the entire wall thickness of thin portions of the casting. This result may be extremely detrimental to the mechanical properties needed for the particular application.
Both solutioning of certain second phase particles and the presence of residual casting stresses encourage the undesirable grain growth. Since solution of certain second phase particles is an essential step in developing the required mechanical properties, the elimination of casting residual stresses must be accomplished if excessive grain growth is to be avoided.
In most conventional alloys, stress relief heat treatments are conducted at temperatures well below those at which grain growth occurs. However, in superalloys the residual stresses are retained at much higher temperatures, approaching and perhaps even exceeding the second phase particle solvus temperature. Thus grain growth can easily occur during stress relief of superalloy components.
Cox, et al., in U.S. Pat. No. 3,677,830, of common assignee herewith, controlled grain growth in nickel base superalloys after working but before aging by subjecting the alloy to a duplex heat treatment consisting of a first heat treat step establishing a uniformity of the precipitated phase throughout the alloy microstructure under conditions of restricted growth due to the presence of a secondary phase, and a second heat treat step providing uniform solutioning of the secondary phase and controlled grain growth by relying upon grain annihilation under conditions of uniform strain energy distribution within the polycrystalline aggregate. These heat treatments were performed within 25.degree.-100.degree. F. (14.degree.-56.degree. C.) below the secondary phase solvus temperature, and resulted in a uniform, reproducible microstructure from which subsequent aging heat treatments were able to promote maximum alloy strength.
Blackburn, et al., in U.S. Pat. No. 4,820,356, of common assignee herewith, controlled grain growth in superalloy forgings by a three step heat treat process, with the first step being development of coarse grain boundary .gamma.' by a subsolvus solution treatment which puts the majority of the .gamma.' into solid solution but retains a sufficient amount as precipitates in the grain boundaries to prevent significant grain growth. The grain boundary precipitates are retained in the subsequent steps and effectively control grain growth.
Gostic, et al., in U.S. Pat. No. 5,074,925, of common assignee and sharing a common inventor with the present application, solution heat treated cast single crystal superalloy materials followed by forging or rolling steps at working temperatures about 50.degree.-300.degree. F. (28.degree.-167.degree. C.) below the .gamma.' solvus temperature. It was found necessary to relieve the residual stresses due to the deformation processes in order to prevent recrystallization of the single crystal material. Gostic, et al., determined that a cyclic annealing process, consisting of several increasing and decreasing temperature cycles within the temperature range of 50.degree.-125.degree. F. (14.degree.-69.degree. C.) below the Y' solvus temperature, alternated with the deformation steps, was effective in preventing the undesirable recrystallization.